Mechanical response of tendon subsequent to ramp loading to varying strain limits

Objective. To determine the in vitro elastic limit of avian flexor tendons utilizing more current methodologies. Design. Assess the mechanical changes between subfailure and subsequent failure ramps at various loading levels. Background. Currently accepted values of elastic strain limits were determined utilizing older methodologies. Consequently, reported values are fairly small and imply matrix damage occurs with small strains. Methods. Flexor tendons were loaded in vitro, to various subfailure strain levels between 1% and 14%, allowed to rest for 5 min, then taken to failure. Suture markers, across the midsubstance, and cryo-grip displacement were monitored for strain using a video strain analysis system and a linear variable displacement transducer, respectively. The mechanical changes between the various subfailure and failure ramp loadings were assessed. Results. Varying strains of subfailure ramp loading did not influence (P > 0.05) the ultimate tensile failure strength, elastic modulus, strain at failure, or strain energy density of tendons. In addition, residual strain after subfailure loading was not significant, nor was it influenced by the level of the subfailure loading. Conclusions. Tendon behaves elastically under ramp loading to significantly higher strains (nearly failure) than previously reported (4%). Relevance This study has found strain thresholds required to cause matrix damage to be significantly higher than previously thought, implying that matrix changes to acute loading events are more a result of the cellular response to the loading event. The clinical relevance is that the clinician may have a greater opportunity to prevent matrix changes than once thought by biochemically altering the cellular response. (C) 2003 Elsevier Ltd. All rights reserved.